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Tuesday, 13 June 2017

Why Won’t My Motorcycle Engine Rev To Redline?

Dear MOby,

I dragged home an ’82 Suzuki GS1100E from a friend of a friend’s basement a couple months ago. At first, it appeared way dusty and sad and I thought I was being taken. But after not all that much cleaning and polishing, it’s a really terrific looking old motorcycle nobody else has, and I’m really liking it.

I don’t know how long it had been sitting. There was no gas in the tank, but some terrible ex-petroleum semi-liquid did dribble out of the float bowl drains. I took the carbs apart and the jets were all actually totally plugged with dried-up residue. I took them to a local shop and had them cleaned in one of those ultrasonic tanks. Put in new spark plugs and air filter, and inspected all the intake area for leaks. Everything seems to be good, gas is flowing through the petcock, spark is good to all four cylinders. It’ll start up, idle, and run pretty okay up to 5,000 or so rpm – but past there, it spits back and runs rough and refuses to rev any farther. What’s the problem?

Revless in Seattle

Dear Revless,

Mickey Cohen of Mickey Cohen Motorsports and right-hand man Cyle Winkler tell us what’s often the malady in situations like this is that your main jets have actually been etched by the nasty old brew that sat in there, a sort of PTSD for carburetor brass parts. In the old days, it was a no-no to drill jets out because they’re much more than just brass plugs with holes in them; those holes are actually carefully shaped for smooth fluid flow.

Shiny and new…

The scars left behind by the old gas in those soft brass nozzles, especially if ethanol was the last fuel in there and especially if they were plugged solid, actually disrupts the smooth flow of gas through the carburetors and into the engine. MC suggests ordering up new jets, both main and pilot, and replacing the float valve needles and seats while you’re at it, since they’re also brass. Good luck!